This invention relates generally to the forming of metal parts and particularly to the in press measurement of the thickness of the workpiece from which parts are formed.
The quality of parts made by forming thin sheets of metal, such as the parts for the electron gun of a color television kinescope, is highly dependent upon the thickness of the material from which the parts are formed. Deviation in the metal thickness from the nominal value results in changes of the forming force and pressure pad force. These variations in forces produce parts for which the bending, spring back and wall thinning are very difficult to predict and compensate. Additionally, changes in the forming forces contribute to excessive wear and fatique of the dies from which the parts are formed, and thus substantially reduce the life of the dies.
Currently, the metal forming industry measures the thickness of the stock based upon a discrete quality control scheme. Typically, samples from lots are selected and the thickness of the samples measured. The entire batch of metal is thus either accepted or rejected based on measurements of the samples. Typically, the samples are selected from the ends of the roll and therefore the sample measurements are not indicative of the thickness of the metal in the middle of the roll. For these reasons, using the sampling method, it is possible to accept a defective roll of material and also to reject an acceptable roll of material. The metal forming industry is starting to measure the thickness of the incoming stock on a continuous basis before the stock enters the die. Typically such measurement is accomplished by a noncontact transducer of the capacitive type. This noncontact type of measurement suffers from a disadvantage in that impurities, or lubricants can change the dielectric and render the measurement inaccurate. Additionally after the metal enters the die, the capacitive transducer provides no information regarding lubricant thickness or impurities that may have adhered to the metal subsequent to the measurement and hence modified the effective thicknesses. For this reason, there is a need for a system for measuring the metal thickness in the die under actual working conditions. The instant invention fulfills this long felt need.